Many communication systems, both wired, e.g., Ethernet, and wireless, e.g., HF, VHF, or UHF, use a form of Carrier Sense Multiple Access (CSMA) to determine whether a radio frequency or communications channel is used by another station before using that frequency or channel. This technique does not work well in a noise- and interference-rich environment. To overcome this problem, some communications systems intersperse a training sequence into modem waveforms to assist in signal detection and channel tracking. In standard HF modem waveforms, however, signal detection is hampered by the use of heavily filtered Phase Shift Keyed (PSK) waveforms, e.g., M-PSK, which are difficult to distinguish from background noise and interference. Without a training sequence, the ability to determine the presence or absence of these waveforms is difficult.
Commonly assigned U.S. Pat. No. 7,593,488, the disclosure of which is hereby incorporated by reference in its entirety, addresses the problem of determining the presence or absence of a PSK signal without requiring a training sequence. That disclosed communications system collects a plurality of signal samples at a selected frequency for a known period of time, determines a metric dependent on at least one measured characteristic associated with the collected signal samples, and indicates signal status when the determined metric is greater than a known value. The signal status is indicated as absent when the determined metric is less than a known value. That system operates with time-domain samples and calculates the mean-square and variance of the magnitude when in-phase (I) and quadrature (Q) samples are received.
Some communications systems operate well when using the techniques disclosed by the '488 patent. Simultaneous transmission (collision) by radio systems that do not use some form of automatic repeat request (ARQ) or reception acknowledgement will typically fail. The radio user also has no visibility into this failure and would not be able to determine its cause. This technical problem is especially relevant in satellite communications. UHF military satellite communications (MILSATCOM) use frequency selective 5 kHz and 25 kHz channels that are filtered and hard-limited, allowing only a single transmission at a time. As a result, the technique described in the '488 patent does not operate well with MILSATCOM and similar communications systems. Line-of-sight (LOS) communications for FM voice, AM voice and all single frequency, non-spread spectrum communications have a similar limitation.